nVidia just released a new set of drivers and they are specifically aimed at GTA IV players. The success you'll have with this software is a bit hit and miss - some users are reporting vast performance increases while others say there is no change. Trying won't hurt, though. nVidia users can get the new drivers here.

A new ATi driver will be released on December 10th and should significantly improve performance for gamers with those cards. Also, another note from Rockstar Games states what we should expect from the coming patch:

Patch Is On The Way
We are working on making a patch available in the next few days. Since Grand Theft Auto IV is a Games For Windows – Live game the patch must be certified by Microsoft before release. The patch is already at Microsoft and we expect a speedy approval. The patch contains a variety of fixes including:

A fix to the crash after legal screen that some German customers were reporting.

Support for DirectInput controllers. Note: The current hack way of supporting these controllers may not work with the improved functionality. So we recommend you remove the hack before upgrading to the patch.

I eagerly await rockstar's patch. I don't really know how much I can expect to gain just from a driver update from ATi.

edit: also for those with ATi cards, I found these over at HardForum. They're betas of the new driver's to be released. I've heard performance gains in GTAIV are minimal, but present. I'm installing right now to see first hand.

On another note, I installed the game on my roommate's computer to be greeted with the missing textures bug. Looks like he'll be needing to upgrade the driver's for his 9500GT too. The game runs surprisingly well on his cheap system though. Maybe because there weren't textures to load. lol